ALIMENTARY TRACT MOVEMENT 97 



tary tract is ready for use by the living cells. But in 

 order to reach them it must pass from the alimentary 

 tract into the blood, to be carried by the blood to all parts 

 of the body. The process by which it gets from the 

 alimentary tract into the blood is called absorption. It 

 consists in the passage of the material through the very 

 thin membrane, consisting of one layer of cells, which 

 lines the alimentary tract, and through a second thin 

 membrane which makes up the wall of the capillary. 

 This process of absorption takes place almost wholly in 

 the small intestine. In essence, it is the same process 

 described in Chapter VI as the means by which substances 

 pass through plant membranes. Whenever the intestine 

 contains digested food in any quantity the concentration 

 of these foodstuffs within it is higher than in the blood, 

 and consequently foodstuffs diffuse from the intestine 

 into the blood. The materials which enter the blood 

 in the intestinal capillaries leave it shortly in the capil- 

 laries of the tissues which require nourishment, so the 

 blood does not have a chance to become loaded with 

 foodstuffs to an extent which would interfere with 

 absorption. 



